Wind turbine technology is providing an ever-increasing proportion of the electrical energy generated in the United States and worldwide. Typically, at least in the United States, wind farms have been located in rural or sparsely populated areas. However, with an increasing demand for the renewable energy represented by wind turbine generators, it is likely that wind farms will in the future be situated closer to populated areas than they are today. When that happens, nuisances created by wind farms will become problematical as people discover that there are esthetic impediments and practical drawbacks to living or working in close proximity to a wind farm. Chief amongst these is the fact that wind turbines are noisy.
Studies conducted to determine both the level of noise generated by wind turbines, and the frequencies and pervasiveness of such noise, provide startling conclusions that wind turbine noise is a significantly greater annoyance to humans than previously suspected. Utility wind turbine generators, for example, can develop noise in the range of 90-100 db at or near the base of the tower. While some of the noise of a wind turbine generator is based upon mechanical factors such as gear boxes, generators, and turning parts, a significant portion of the noise is due to aerodynamic factors related to the shape of the blades that catch the wind.
Wind turbine blades are airfoils that provide lift by means of the differential in pressure as air flows over the blades. The differential pressure and subsequent lift is caused because air passing over the top of the blade travels faster than air traveling over the bottom of the blade. As the air converges at the trailing edge of the blade the differing air velocities cause vortices. Vortices are whirling air disturbances that maintain their general shape until the energy within them dissipates, at which point they collapse, resulting in an implosion of air that raises sound pressure levels, or noise. Regular vortices that form at the trailing edge of an aerodynamic surface such as, for example, a helicopter main rotor blade, create distinctive sounds as they collapse. The same phenomenon causes wind turbine generators to make noise as the blades rotate in response to wind. Where the trailing edge of an airfoil is substantially straight, vortices of approximately equal strength will develop as air passes over the airfoil, and will tend to collapse substantially simultaneously at a given distance behind the trailing edge. The sound made by the collapse of individual vortices is reinforced through the simultaneous collapse of adjacent vortices created by the same trailing edge, with the result that appreciable and potentially annoying noise is generated behind each blade of a rotating wind turbine.
A number of techniques have been employed to reduce or change the pattern of vortices caused by air confluence at trailing edge of the blade, and thereby reduce noise emissions. One such method is to add serrations to the trailing edge of the blade in order to reduce the vortices thereby reducing the sound pressure level. The serrations are generally equally spaced serrations of the same size positioned along the trailing edge of the blade. See, e.g., U.S. Pub. No. 2009/0074585 to Koegler et al.; U.S. Pat. No. 7,059,833 to Stiesdal et al.
The major problem with the current technology however is that although serrations may be effective in reducing overall noise levels, the resulting frequency and amplitude of vibration remain as a constant repeating pattern. What is needed is a means to reduce aerodynamic noise generated by commercial wind turbine blades without significantly reducing blade efficiency or adding to the weight or cost of the unit.